Instruments for the measurement of infrared (IR) radiation are becoming increasingly important for a variety of commercial and non-commercial applications. Research into the development of uncooled sensors with response throughout the infrared spectrum has been particularly important due to the limitation on the operation of cooling systems. Uncooled infrared sensors would have important applications for space-based remote-sensing of thermal sources, night vision, target identification, thermal mapping, event detection, motion detection, and others. The limitations of the performance of the existing uncooled sensors often are the primary constraints to the performance of infrared imaging systems for many applications. As a result, there has been considerable investment in the development of uncooled infrared sensors.
A broad assortment of infrared detectors has been developed over the last 40 years. In most cases, they may be classified as either quantum or thermal detectors, depending upon whether the incoming radiation is converted to excitations which are collected, or is converted to heat and detected through changes in temperature. In general, a quantum detector which operates at detector temperatures Td is usually superior to a thermal detector at the same temperatures for infrared frequencies in which hv&gt;&gt;k.sub.B T.sub.d, where h is Plaiick's constant and k.sub.B is Boltzmann's constant. However, for infrared frequencies in which hv&lt;&lt;k.sub.B T.sub.d. thermal detectors represent the only functional technology. The operation of quantum detectors is limited by the availability of efficient photon conversion mechanisms, while the operation of thermal detectors is limited by the availability of sensitive thermometers. Only thermal infrared sensors operate in the mid-to-far infrared range (.lambda.&gt;10 .mu.m) at room temperature.
The pneumatic infrared detector, which was originally developed by Golay, is classified as a thermal detector. Golay's detector consists of a small cavity filled with gas at room temperature. The cavity is separated from the surroundings by a window and a thin, flexible membrane. The membrane is coated on one side with a thin metallic film, which has significant absorption throughout the infrared spectrum whenever the sheet resistance of the film is approximately half of the impedance of free space. The trapped gas in the Golay cell is heated by contact with the membrane and expanded thermally, which forces the membrane to deflect outward. This deflection is usually detected with optical or capacitive displacement transducers. At present, these detectors are bulky, fragile, difficult to fabricate, and expensive. Nevertheless, they have been widely used, primarily because of their improvement in sensitivity over all other room-temperature detectors in the mid-to-far infrared range. Attempts to miniaturize the Golay cell for incorporation into focal plane arrays have been unsuccessful because of scaling laws which relate the sensitivity of conventional displacement transducers and their active area. The need for focal-plane arrays of uncooled detectors stimulated the development of pyroelectric detector arrays, the best of which are 5-10 times less sensitive than the Golay cell.
Current state-of-the-art uncooled IR focal plane arrays use many different thermal detection mechanisms such a bolometric (sensor resistance is modulated by temperature), pyroelectric (dielectric constant is modulated by temperature), and thermoelectric effects. As discussed above, thermo-mechanical effects have been explored using modifications of the Golay cell. The performance of IR imagers based on these technologies is limited compared with imagers based on direct photon conversion, such as PtSi detectors operated 77 K, and also is considerably worse than the theoretical background limited performance. In all approaches, the fundamental limits to the performance are controlled by the ability to thermally isolate the detector from its surroundings, the detector sensitivity to a change in temperature, and the introduction of extraneous noise sources. One of the reasons for degraded performance is the parasitic thermal resistance paths inherent in the supporting structures of the sensing elements. Another reason is the electronic noise present in the readout scanning circuitry.
With the above considerations in mind, the present invention is based on the development of an IR capacitance structure that deflects the position of a plate in response to temperature changes.